Variable displacement hydraulic pumps and motors are often used as a rugged, reliable and convenient way to transfer drive shaft power in a controlled manner. Such hydrostatic drives are used in construction vehicles and equipment, agricultural machinery, materials handling equipment, maritime vessels, machine tools, garden tractors and recreational vehicles.
In a typical application, a variable displacement pump is driven by a power source, such as a diesel or gasoline engine, turbine or electric motor. Flexible hydraulic lines or hoses connect the pump output to a hydraulic motor that drives the load.
In one known prior art form of hydrostatic drive, the pump was a variable-displacement piston pump having a pivotal swashplate for determining the length of stroke of a pump piston. The angle of this swashplate was set by stroker pistons controlled from an electrical command signal to an electrohydraulic servovalve which had an output stage comprising separately and relatively movable spool and sleeve valving members to control fluid flow with respect to such stroker pistons. The position of the stroker pistons determined the angularity of the swashplate and hence the displacement of the pump. A mechanical connection was made between the swashplate and the valve sleeve to provide one-to-one follow-up feedback with respect to the valve spool. In this manner, an electrical input to the electrohydraulic controller commanded a proportional displacement of the valve spool which caused a hydraulic output to the stroker pistons to create, ultimately, swashplate position and pump displacement proportional to the electrical input.
Such an arrangement having a mechanical feedback mechanism between the pump swashplate and the output stage of the servovalve is disclosed in the U.S. patent application of John T. Caruso, entitled "Feedback Mechanism For Variable Displacement Hydraulic Device Having An Electrohydraulic Controller", signed by him on Jan. 11, 1978, further identified by Ser. No. 869,829 filed Jan. 16, 1978, and owned by the assignee of the present application. The disclosure of said application Ser. No. 869,829 is incorporated herein by cross-reference thereto.